As a transducer that performs transmission/reception of ultrasound, a CMUT (Capacitive Micromachined Ultrasonic Transducer), that is a capacitive ultrasound transducer manufactured by a MEMS (Micro Electro Mechanical Systems) process to which a semiconductor process is applied, has been proposed. There has been a measurement method by which ultrasound is transmitted to a measuring object, and the ultrasound reflected by the measuring object is received to obtain the information about the measuring object, with the use of an ultrasound transducer. There has been a suggested method by which the transmission/reception transducer used in the above measurement method is a CMUT that characteristically has a relatively wide frequency range (wideband) of ultrasound to be transmitted/received.
It is known that a CMUT has two modes in which the condition of a diaphragm that vibrates upon receipt of ultrasound varies: a conventional mode and a collapse mode (see PTL 1). In the conventional mode, the diaphragm is not in contact with a lower electrode during a transmitting/receiving operation, even if the diaphragm is concaved. In the collapse mode, on the other hand, the diaphragm is brought into contact with a lower electrode when concaved during a transmitting/receiving operation. PTL 1 discloses that ultrasound with large amplitude can be emitted by causing a diaphragm to vibrate between the conventional mode and the collapse mode.
According to a method of measuring ultrasound through transmission and reception, the required frequency range varies with measuring objects. Examples of typical features include a feature centering around 5 MHz (about 1 MHz through 5 MHz, for example) and a feature centering around 10 MHz (about 7 MHz through 12 MHz, for example). As an example of the former feature, there is a method of receiving ultrasound generated from a measuring object after light emission. This is also called photoacoustic tomography (PAT). As an example of the latter feature, there is a method of receiving ultrasound reflected by a measuring object after ultrasound transmission. As such, the frequency range required for a capacitive transducer varies depending on the size of the measuring object, the depth of the measurement, or the measurement method.